1. Field of the Invention
The present invention relates to a proton conductor, a production method thereof, and an electrochemical device using the proton conductor.
2. Description of the Prior Art
In recent years, as a polymer solid-state electrolyte type fuel cell has been used to power cars, there has been known a fuel cell using a polymer material having a proton (hydrogen ionic) conductivity such as a perfluorosulfonate resin (for example, Nafion® (produced by Du Pont).
As a relatively new proton conductor, there has also been known a polymolybdate having large amount of hydrated water such as H3Mo12PO40.29H2O or an oxide having a large amount of hydrated water such as Sb2O6.5.4H2O.
The above-described polymer material and hydrated compounds each exhibit, if placed in a wet state, a high proton conductivity at a temperature near ordinary temperature.
For example, the reason why the perfluorosulfonate resin can exhibit a very high proton conductivity even at ordinary temperature is that protons ionized from sulfonate groups of the resin are bonded (hydrogen-bonded) with moisture already entrapped in a polymer matrix in a large amount, to produce protonated water, that is oxonium ions (H3O+), and the protons in the form of the oxonium ions can smoothly migrate in the polymer matrix.
More recently, there has been also developed a proton conductor having a conduction mechanism quite different than that of each of the above-described proton conductors.
That is to say, it has been found that a composite metal oxide having a perovskite structure, such as, SrCeO3 doped with Yb, exhibits a proton conductivity without use of moisture as a migration medium. The conduction mechanism of this composite metal oxide has been considered such that protons are conducted while being singly channeled between oxygen ions forming a skeleton of the perovskite structure.
The conductive protons, however, are not originally present in the composite metal oxide but are produced by the following mechanism: namely, when the perovskite structure contacts the steam contained in an environmental atmospheric gas, water molecules at a high temperature react with oxygen deficient portions which have been formed in the perovskite structure by doping Yb or the like, to generate protons.
The above-described various proton conductors, however, have the following problems.
The matrix material such as the above-identified perfluorosulfonate resin must be continuously placed in a sufficiently wet state during use in order to keep a high proton conductivity.
Accordingly, a configuration of a system, such as, a fuel cell using such a matrix material, requires a humidifier and various accessories, thereby giving rise to problems in enlarging the scale of the system and raising the cost of the system.
The system using the matrix material has a further problem that the range of the operational temperature must be limited for preventing the freezing or boiling of the moisture contained in the matrix.
The composite metal oxide having the perovskite structure has a problem that the operational temperature must be kept at a high temperature of 500° C. or more for ensuring an effective proton conductivity.
In this way, the related art proton conductors have the problems that the atmosphere dependence on the performance of each conductor becomes high, and more specifically, moisture or stream must be supplied to the conductor to ensure the performance of the conductor, and further, the operational temperature of the conductor is excessively high or the range of the operational temperature is limited.